1. Field of the Invention
This invention relates to an apparatus for retrieving a track on a recording medium by a light beam. For example, it relates to an apparatus, in an optical or magneto-optical recording/reproducing system, in which a predesignated one of a large number of tracks on the recording medium may be located by a light beam at an elevated speed during signal recording, reproduction or erasure. Above all, when the beam shifting or movement for sweeping is controlled on the basis of feedback control by the track traverse velocity during the time the light beam is moved to the pre-designated track, an improvement on the velocity detection circuit for detecting the track traverse velocity is comprised within the technical field of the present invention.
2. Description of the Prior Art
An example of this type of the known track retrieval apparatus is disclosed in the JP Patent KOKOKU Publication No. SHO-63-46511.
The outline of this conventional track retrieving apparatus is hereinafter briefly explained. A movable carriage mounts an optical system for collimating the light from a light source, such as a laser diode, and outputting the collimated beam. By relative movement of the carriage with respect to the recording medium, the recorded or unrecorded tracks on the recording medium are swept in the track traversing direction by the outgoing light beam from the optical system.
On the other hand, the reflected light of the light beam reflected by the recording medium is detected by a photodetector provided on the carriage. The detected signal is processed in a predetermined manner, as described subsequently, for effecting feedback control of the velocity of the carriage movement for irradiating the light beam on one predesignated track, that is, for retrieving the designated track.
Meanwhile, the medium is in the form of a disk or a rectangle, while the tracks are in the form of straight lines, concentric circles or helices.
Throughout the present specification, the number n of the tracks traversed by the light beam means the number of tracks traversed by the light beam travelling in the direction normal to the tracks or in the radial direction of the recording medium when the tracks are in the form of straight lines or concentric circles, and the number of times the track is traversed by the light beam travelling along the radius of the track when the tracks are in the form of helices. The number of tracks means the number of separate tracks when the tracks are in the form of straight lines or concentric circles, and the number of turns of the helix, that is the number of the tracks each of which is assumed to exist from a given point on a track to a separate neighboring point on the adjacent track.
Meanwhile, for the above mentioned control of the carriage velocity it is necessary to detect the velocity with which the light beam traverses the track. With the above described track retrieval apparatus, there is employed, as a means for detecting the track traversing velocity, a combined method consisting of a differentiating detection system which detects the number of tracks or number of times traversed by the light beam on the basis of the photodetection output and differentiates the detected number with respect to time to detect the track traversing velocity and a magnetic detection system for magnetically detecting the carriage velocity.
For detecting the carriage velocity by the magnetic detection system, a velocity detection unit composed of a movable section movable in unison with the carriage and a stationary section fixed at a suitable point in the apparatus is annexed to the carriage. The relative velocity between the movable section and the stationary section stands for the carriage velocity, that is the track traversing velocity of the light beam. The relative velocity between the two sections is taken out from the velocity detection unit as the magnetic signal which is subsequently converted by a suitable converter into an electrical signal.
For velocity detection by the differentiating detection system, a change in the volume of the reflected light when the light beam traverses a track is detected by the photodetector, from the output of which a pulse is generated each time the light beam traverses a track (one pulse per track). The number of times n the light beam traverses the track may be measured by counting the number of the generated pulses. The number of times n of traverse is converted into an analog signal which is then differentiated with respect to time by a differentiating circuit to detect the track traversing velocity of the light beam.
The detected velocity values from the magnetic detection system and the differentiating detection system are selectively switched depending on, for example, the magnitude of the detected velocity, so as to be taken out as the current velocity signal indicating the current track traversing velocity, which current velocity signal is supplied to a driving circuit. This driving circuit transmits a carriage control command signal to a driving element so that the difference between the current velocity signal and a separate command velocity signal will be zero on an average, the driving element in turn controlling the carriage velocity on the basis of the command signal supplied thereto.
The above described prior art system suffers from the following disadvantages.
First, in the magnetic detection system, a velocity detection system for magnetically detecting the travelling velocity of the carriage is necessitated in addition to an electrical circuit. This type of the magnetic velocity detection unit occupies a larger space and is relatively expensive to render the reduction in size and costs of the retrieval apparatus and hence that of the recording/reproducing apparatus difficult.
On the other hand, with the differentiating detection system, the rate of pulse generation for velocity detection is perpetually one pulse per track, irrespective of the velocity. In such a case, for a low velocity range in which the period of the track traversing velocity becomes longer, a higher detection accuracy may be achieved due to the long sampling time duration for pulse generation, whereas, however, ripples tend to be inconveniently produced as a result of the longer sampling time duration. Conversely, for a high speed range in which the period of the track traversing velocity becomes shorter, the detection accuracy is lowered due to the shorter sampling time interval. In addition, the detection accuracy for the low velocity range is not identical with that for the high velocity range due to the difference in the period of the track traversing velocity which renders it difficult to improve the detection accuracy in the broad velocity range.